Phalanger
- An open source PHP compiler written in .Net.
- http://php-compiler.net/doku.php

OpenXML Writer
- OpenXML Writer is an opensource text editor for creating WordprocessingML files (.docx) that is created with the .Net Framework.
- http://www.openxml.biz/OpenXMLWriter.html

Paint.NET
- An open source photo editor that teaches many aspects of Windows forms development. Written entirely in C# the source code is under an MIT open license agreement.
- http://www.getpaint.net/

Passage .Net - Open Source .NET Portal Software
- An open source enterprise portal and content management system written in C# and has a BSD license.
- http://www.cubiccompass.com/

SharpDevelop
- An open source integrated development enviroment written in C#.
- http://www.icsharpcode.net/OpenSource/SD/

NUnit
- An unit testing framework that is open source written in C#.
- http://www.nunit.org/

mojoPotal
- mojoPortal is an open source content management system and web site framework portal for .NET.
- http://mojoportal.com

MonoUML
- The MonoUML Project aims to develop a CASE tool for GNU/Linux and UNIX environments, using Mono as development and running framework.
- http://www.monouml.org/

Piccolo Reserach Project
- A toolkit that supports the development of 2D structured graphics programs allowing for Zoomable User Interfaces (ZUIs).
- http://www.cs.umd.edu/hcil/piccolo/learn/about.shtml

Spring.NET - Application Framework
- An application framework that provides capabilities like dependency injection and Aspect Oriented Programming (AOP) for building enterprise .NET applications.
- http://www.springframework.net

Skelta - Workflow
- A embeddable .NET based workflow framework
- http://www.skelta.com/

VNC-Client for .NET
- A VNC-Client written in C# using the .NET Framework. VNC is a remote display system, which allows the user to access the desktop of a remote machine. [Open source, GPL]
- http://dnvnccl.sourceforge.net/

DataDesigner
- Standards based .Net document server. With peer-to-peer content exchange abilities. [Public Domain]
- http://sourceforge.net/projects/datadesigner/